SS jFet (pre)amp

[TejfolvonDanone]

true, it is how the fetzer valve is built up by copying old tube amps.

Example of kits available that use the same input concept:

Does the "it was like this before" makes it good nowadays? E.g. Any leakage or grid current in an amp + a true bypass pedal makes a pop whenever you turn the pedal on/off. It wasn't a problem when there weren't any pedals but nowadays it's a whole different story.

I am afraid i think it works differently. Power sag is much slower than a wave fragment. That's why it compresses, instead of just adding clipping. Just as .... in a compressor. The voltage drop only occurs after an amount of current draw raise, when the capacitor is getting discharged. This attack and decay of the sag is determined by the impedance and capacitance time constant. A zero power source impedance woud lead to no sag at all. An infinite capacitance would lead to infinite time constant (no current draw would change the voltage held by the cap).
in other words: the voltage drop occurs after the signal draws more current and this with a certain time span. This changes the headroom and the bias of all stages as well. If the voltage drops, the voltage between the two bases of the class AB stage drops as well, possibly below 2 x Vbe(cut off).....

I'm afraid you are right But in needs to sag a lot to have a noticeable effect. I believe that you won't have enough sag from 18V to have a clean output with just added crossover distortion.

Also the values you used in the supply line need some tweaking. The sag and (signal frequency) ripple depends on the resistance and capacitance and not just time constant. You get really different sag and ripple if you keep the time constant same but increase the cap.


I noticed that on your design the JFET source follower has contradictory conditions. Namely the Vgs(off) of the FET and the 0.5mA bias. The FET has a minimum -1.5V Vgs(off) so it won't bias with 0.5mA ever.

Yet all it does is clipping. I haven't seen non-linear opamp stages (yet).

A tube screamer is actually a logarithmic amplifier of sort. Which is in fact a non-linear op-amp stage.

[Steben]

Does the "it was like this before" makes it good nowadays? E.g. Any leakage or grid current in an amp + a true bypass pedal makes a pop whenever you turn the pedal on/off. It wasn't a problem when there weren't any pedals but nowadays it's a whole different story.

No not good by definition. I might add a cap. I might not. of course popping exists in pedals which aren't designed correctly on thier own (discharge resistor....) .

I'm afraid you are right But in needs to sag a lot to have a noticeable effect. I believe that you won't have enough sag from 18V to have a clean output with just added crossover distortion.

I don't want clean with crossover I want crossover with the clipping.

Also the values you used in the supply line need some tweaking. The sag and (signal frequency) ripple depends on the resistance and capacitance and not just time constant. You get really different sag and ripple if you keep the time constant same but increase the cap.

True, with lower resistor => less voltage drop..... :/

I noticed that on your design the JFET source follower has contradictory conditions. Namely the Vgs(off) of the FET and the 0.5mA bias. The FET has a minimum -1.5V Vgs(off) so it won't bias with 0.5mA ever.

J201 can have very low Vgs off

A tube screamer is actually a logarithmic amplifier of sort. Which is in fact a non-linear op-amp stage.

I mean non-linear transfer curve in the non clipping region. Assymetrical giving even order harmonics content without clipping.